Various methods for synthesizing alkylene-glycols, particularly ethylene-glycol, have been proposed in the literature.
The Japanese patent No. JP-A-52-42809, in particular, discloses a method for synthesizing ethylene-glycol from a gas mixture containing carbon monoxide and hydrogen, in the presence of a rhodium containing catalyst. However this method requires a high pressure of about 50 megapascals (MPa) or more and, accordingly, is not very attractive and difficult to industrially develop.
Another method for synthesizing ethylene-glycol has been disclosed in U.S. Pat. No. 4,087,470.
This method comprises successive steps of first converting formaldehyde to glycolic acid by carbonylation reaction with carbon monoxide, then converting glycolic acid to glycolate by esterification and, finally, hydrogenating the latter to glycol.
This method is also difficult to industrially develop, the formaldehyde carbonylation step requiring high pressures and being performed in the presence of a highly corrosive acid catalyst such as hydrofluoric acid.
It has also been proposed to produce ethylene-glycol and ethylene-glycol ethers directly from formaldehyde by reaction with the synthesis gas in the presence of rhodium or cobalt and rhodium containing catalyst (U.S. Pat. Nos. 4,079,085, and 4,144,401) the reaction taking place in the presence of an alcohol as solvent under a pressure of about 20 MPa. The selectivity to ethylene-glycol is low and the yield of free and monoetherified ethylene-glycol is not higher than 35% with respect to formaldehyde.
Recently acetals of formaldehyde or acetaldehyde have been used as starting materials for producing, in particular, ethylene-glycol or propylene-glycol monoethers by reaction with the synthesis gas, in the presence of a catalyst containing cobalt carbonyl derivatives. These monoethers may then be hydrolyzed in acid medium to the corresponding glycols.
The reaction equation can be written: ##STR1## wherein R.sub.a is hydrogen or a methyl group and R.sub.b an alkyl group. The British Pat. No. GB-A-2 070 002 discloses the reducing carbonylation of formaldehyde acetals to glycol monoalkyl ethers which are then hydrolyzed to glycols.
The total yield to ethylene-glycol amounts to 76%; however the operating conditions used in the reducing carbonylation step are severe (temperature of 180.degree.-190.degree. C. and pressure of 20-30 MPa) and constitute a substantial disadvantage of said process.
The reducing carbonylation of acetaldehyde acetals has not been studied thoroughly and requires severe operating conditions for relatively low yields of propylene-glycol ethers, as shown in U.S. Pat. Nos. 4,356,327 and 4,390,734.